A genetic research program is proposed to investigate mammary tumor progression in mouse models generated by transgenic and gene targeting approaches, including a novel knock-in strategy that we have developed for conditional, tissue-specific overexpression of oncoproteins. Specifically, tumor development will be examined in compound mutant progeny derived from crosses between various genetically-modified mice that are tumor-prone because of overexpression of oncogenes or lack of tumor suppressors. In this regard, the causal combinatorial involvement of interacting deranged signaling pathways that are engaged in the progression of marmnary carcinomas by promoting unregulated proliferation, reducing apoptosis and potentially causing genomic instability will be analyzed using a set of assays that include expression profiling with DNA microarrays. The pathways involving the c-Myc and ErbB2/Neu genes, which are frequently amplified and/or overexpressed in human breast cancers, were chosen as focal points of the investigation, to keep the proposed project pertinent to the human disease. Among other questions that will be addressed, including the potential emergence of genomic instability in the absence of the Brcal tumor suppressors, signaling will be investigated in relation to loss- or gain-of-function genetic modifications affecting apoptosis, such as caspase inhibition, p53-mediated tumor suppression and effects of the components of the PI3-kinase/Akt1 major antiapoptotic pathway that involves PTEN as a central negative regulator and is associated with signaling through the IGF receptor. In parallel, the identification of new synergistic interactions between oncogenic pathways contributing to mammary tumor progression in transgenic mice will be sought through the development of an approach for insertional somatic mutagenesis using transposon tagging. The proposed studies, performed advantageously in the context of the entire experimental organism, have the potential to establish causal relationships between evcnts in the multistep process of mammary carcinogenesis, and are likely to provide information of practical significance towards the identification of candidate targets for rational therapeutic intervention to treat breast cancer.